text:
What does it take to turn a 747 around 180 degrees?
Assume that a 747 is flying at cruising altitude and speed. If the pilot decided (for a presumably good reason), to turn the plane 180 degrees,

How quickly could the turn be completed?
How wide would the turn radius be?
How much altitude would be lost, if we're not assuming a level turn?

EDIT: I learned something new from both of the great answers below. I accepted the one that was easier to follow. Since both answers assumed a level flight, I have added a #3.

I don't think he's asking about a standard turn rate, but rather how fast it could be turned around, which of course is dependent on speed, bank angle, and whether or not altitude is being maintained. This should be clarified in the question.

If I was at altitude and suddenly realized we were about to cross into airspace that might get us shot down, I would, smoothly, do 3 things simultaneously: (1) start the nose down to around 25 degrees down, (2) start a roll to a bank angle of 60-80 degrees, and (3) start the power back to idle. At the same time, I'd ask the first officer to deploy the speed brakes, which would give me great roll response.

@Terry: Great comment! This needs a few 1000 feet of altitude, but will produce the fastest heading change. The answer is different when the pilot wants to stay at the same altitude, and then I think this question is NOT a duplicate.

@Paul - Hidebound pansies or not, I have over 3000 hours on type and can confirm that the 747 cannot take the negative G (no matter how small the %) required for the half roll to initiate a split-S turn. The Barrel roll that Tex did during the demos of the 707 is a "positive G" maneuver where the airframe remains in a state of positive G throughout. The half roll will bring you into a negative G state and the subsequent recovery to level flight will exceed the airframe limits of the 747. Its a people carrier and not an Extra 300. try it and you will die. I hope you don't work for an airline.

How long, and how wide of a turn depends entirely on three things:

What is the true airspeed.
What is the bank angle of the turn?
Is level flight maintained during the turn.

Since it's easiest to calculate, and presumably what you intended, we'll assume #3 is true (level flight). However, if you really needed to get turned around fast, turning in a descent will give you a tight turn without the extra g-forces.
Cruise Speed for a 747 depends on altitude, generation, company guidelines, etc, but we'll go with Mach 0.85 for this example, which seems to be in the ballpark. If we say it's cruising at 35,000 feet in standard atmospheric conditions, that comes out to 490 knots TAS.
Next, pick a bank angle. A reasonable bank angle for that airplane with passengers on board is 25°, and you could probably get away with 30°. Anything beyond that will cause passengers to complain. Of course, if it's an emergency you might consider something higher. Just remember, the higher the bank angle, the stronger the g-forces which will be felt by the people inside, and the aircraft itself, which has structural limitations. The G-force can be calculated using:
$$
gforce = \frac{1}{\cos(bank)}
$$

For an example, let's select 25° bank since that's the most realistic.
Radius
Calculate the radius of the turn using this formula, slightly modified from Wikipedia to give nmi instead of feet:
$$
Radius\ of\ turn\ in\ nautical\ miles= \frac{velocity^2}{68579 \times \tan(bank)}
$$
Which gives us:
$$
7.51nmi = \frac{490^2}{68579 \times \tan(25°)}
$$
So the turn itself would be about 15 nautical miles wide (≈ 91,000 feet) not accounting for wind.
Distance Traveled
Using basic geometry, the distance traveled for a 180° turn is $d = r\pi$ (half the circumference of a circle), so:
$$
23.59= 7.51\pi
$$
Giving us 23.59 nautical miles traveled.
Duration
$$
time\ in\ minutes = \frac{distance \times 60}{TAS}
$$
Therefore, at 490 knots (nautical miles per hour) and 25° bank, it would take about 2 minutes, 53 seconds to complete the turn.
$$
2.89 = \frac{23.59 \times 60}{490}
$$
You could go through and calculate the results for any speed and bank angle you want. This is true regardless of the type of airplane in question.

Turning in a Descent
Your edited question asks how much altitude would be lost if it were not a level turn. There is also no single answer to this question because it depends entirely on how you perform the maneuver.
One reason you may have to descend in a turn is because the increased lift results un increased induced drag, and the aircraft engines may not have enough power to compensate, resulting in the decreased airspeed, and possibly a stall. In this case, the equations for calculating the turn radius are exactly the same as in level flight. The amount of altitude lost will depend on the descent rate required in order maintain airspeed, which will vary depending on the engine power available, the weight of the aircraft, and the drag curve at the given speed and AoA. Peter Kämpf's answer gives an example of what this might look like for a 747 in a 1.5g turn.
Another version of a turning descent would be an accelerated descent. The advantage in this case it that it would allow turns to be made without additional g-force. The disadvantage is that you'll be accelerating downwards instead of descending at a constant rate. This can get out of hand very quickly and would only be good for very short duration turns.
To illustrate this disastrous option, let's look at what would happen in a 1g turn at the same 25° and 490 knots TAS. Since the math is more complicated, it'll be easier to talk about this portion in metric units. Here's a conversion table:
1 nautical mile = 1852 meters
1 knot = 0.514444 meters per second
1 foot = 0.3048 meters

First, in order to maintain a constant 1g, the lift vector is simply rotated in the turn (instead of rotated and increased in order to maintain altitude). Therefore, the magnitude of our lift vector will be equal to the acceleration due to gravity near the Earth's surface. We'll round that to 9.8 m/s/s.
Inward Acceleration
Our acceleration towards the center of the turn $a_c$ (the portion of our lift vector pointed inwards instead of up) can be determined using the following equation where $\theta$ is the bank angle:
$$
\sin(\theta) = \frac{a_c}{g}
$$
Solving for $a_c$:
$$
a_c = g\sin(\theta)
$$
Therefore
$$
4.142 = 9.8\sin(25°)
$$
Radius
So 4.142 meters per second per second is how fast, at 25° of bank, and 1g, we'll be accelerating towards the center of the turn. With that, information, along with our known velocity, we can calculate the radius of the turn using this equation where $v$ is our velocity in meters per second, and $R$ is the radius in meters:
$$
R = \frac{v^2}{a_c}
$$
Plug in our numbers:
$$
15341 = \frac{(490 \times 0.514444)^2}{4.142}
$$
This comes out to a radius of 15341 meters (8.28 nautical miles).
Distance Traveled
Now that we have our radius, we can calculate how long it will take us to turn 180°. This part is the same equation as before.
$$
48195 = 15341\pi
$$
Giving us 48195 meters (26 nautical miles).
Duration
Distance divided by velocity gives us duration.
$$
t = \frac{d}{v}
$$
In our case:
$$
191.2\ seconds = \frac{48195}{490 \times 0.514444}
$$
So, it'll take 3 minutes 11 seconds to complete the turn.
Downward Acceleration
The last value we need before we can calculate the altitude lost in the turn is to calculate how fast we're going to be accelerating towards the ground in this turn. Lets first calculate the upward portion of our lift vector:
$$
a_u = g\cos(\theta)
$$
Therefore
$$
8.882 = 9.8\cos(25°)
$$
Our lift vector is accelerating us upwards at 8.882 m/s/s, while gravity is trying to pull us down at -9.8 m/s/s. That comes out to a net vector of -0.918 m/s/s.
Altitude Lost
This part requires a little bit of calculus because our descent rate is accelerating. The integral of acceleration is velocity ($v = at$), and the integral of velocity is distance:
$$
d = \frac{at^2}{2}
$$
So, lets calculate the change in vertical distance (altitude):
$$
-16780 = \frac{-0.918 \times 191.2^2}{2}
$$
So, theoretically, our airplane lost 16,780 meters of altitude (55,052 feet). Of course, since we were only 35,000 feet in the air to begin with, this is rather bad news for us. Besides hitting the ground, you would also come close to risking structural damage to the airframe because in this example, 490 knots is considered the speed over the ground, but the total velocity would be higher (about 596 knots) by the end of the turn due to the sink rate.
You'll also notice that, assuming we had the altitude to lose in the first place, the turn took longer to complete than in level flight. This is because the magnitude of the lift vector was smaller.
You can feel free to experiment with other speeds and bank angles, and you could also experiment with higher g turns (simply replace $g$ in the equations with $2g$ or similar). In some cases higher than 1g, you may actually gain altitude, though it's unlikely a 747 would be able to sustain a high-g maneuver which gains altitude.
As a quick second example, consider a 2g turn with 80° of bank at 400 knots:
$$
a_c = 19.302 = 2g\sin(80°) \\
R = 2193.8 = \frac{400 \times 0.514444}{a_c} \\
d = 6892 = R\pi \\
t = 33.5 seconds \\
a_u = 3.404 = 2g\cos(80°)
$$
Giving a total altitude lost of 3589 meters (11,775 feet) over 33.5 seconds. However, at the end of the turn, you would have a sink rate of 214 meters (703 feet) per second. That's over 42,000 feet per minute. It would likely be possible to recover from that in the altitude remaining, but it would not be pleasant.

Try to calculate the instationary turn with maximum lift and use the missing thrust to calculate the sink rate. This will give more realistic results.

@PeterKämpf I agree that the contrived example is unrealistic in the sense that the maneuver would never be performed like that, but the results I gave seem theoretically accurate. If you never increase lift to match gravity, you'll continue in an accelerating descent. If you assume a constant rate descent, then the equations are exactly the same as in level flight... I should probably point that out in the answer. If that doesn't seem right to you, feel free to correct or give insight. I'm not an aerospace engineer or physicist.

In a descending turn you can pull more gs than in a steady turn, because the energy gained by descending compensates the drag increase over the drag of the steady case. Here we run probably into buffeting limits already in the static case, so there will be a massive drag increase for little gain in lift. But the turn will be tighter if flown just like Terry described it.

@PeterKämpf just so I understand the point you're making, essentially what you're saying is: you could perform a steep turn where upward lift equals gravity, but due to the extra induced drag, the engines wouldn't be able to maintain airspeed; however, you could trade potential energy for airspeed via a constant rate descent in order to perform the maneuver. Correct? Yes, that does seem like a more realistic way to do it. I'll update my answer to bring that up and point people to the example in your answer.

To calculate turn rates, best start with a load factor $n_z$ or roll angle $\Phi$ and calculate all other parameters using these formulas: $$n_z = \frac{1}{cos \,\Phi}$$
Radius: $$R = \frac{v^2}{g\cdot tan\,\Phi}$$
Angular velocity (rad/sec): $$\Omega = \frac{v}{R} = \frac{g\cdot tan\,\Phi}{v}$$
An airliner in cruise will fly close to the local maximum lift, so it will not be able to sustain a steep turn. Pulling more than a few percent of 1 g will cause stronger shocks on the upper wing which will cause a steep drag increase and can even stall the airplane. This is called a high-speed stall. Fortunately, things get better extremely quickly when the airplane slows down a little. If it slows down too much, it will, however, enter into a low-speed stall, because both are separated by a small speed range when flying in low density air at high Mach number.
To give you an idea what load factor is caused by which roll angle, here is a small list:
    Ω       load factor [g]
    0°        1.0
   10°        1.0154
   20°        1.0642
   30°        1.1547
   40°        1.3054
   50°        1.5557
   60°        2.0
   70°        2.9238
   80°        5.7588

I would assume that more than 20° bank will not be possible at cruise. Given that the 747 can fly at Mach 0.85, this translates into 500 kt or 258 m/s in 30.000 ft. The radius for this turn will be 18.65 km or 10 miles. Use the inverse of the angular velocity to calculate the seconds per radian: Flying a 180° turn   ($\Omega = \pi$) will take 227 s or three minutes and 47 seconds.

EDIT: Bret Copeland inspired me to add one more case. Not as extreme as Terry would fly it, but it gives you a tighter turn.
Above the turn was flown without sink. I assume that the 747 can not produce much more lift without running into compressibility problems, which would increase drag massively. If I am not concerned about losing altitude, I can use the energy gain from sinking for compensating that extra drag, and then the calculation would run like this:
Starting from a 1.5g turn with 48° bank, the plane needs to produce 50% more lift, which will only be possible when the pilot accepts heavy buffeting and pitch-down (Mach Tuck). But let's just assume that is possible. I expect that the lift curve will be well past the Mach break, and drag will at least double. Using data from this source, and assuming an aircraft mass of m = 340 t, another P = 9 MW of power are needed per second on top of what the engines supply. This is possible by sinking with $w = \frac{P}{m}$ = 26.6 m/s. This is a flight path angle of $atan(\frac{26.6}{258})$ = 6°. (I would not be surprised if the real number is closer to 10°, but I have no good aerodynamic data available right now).
The turn radius is now only 6.11 km and the 180° turn is completed after 74.4 s = 1 minute and 14 s. The altitude loss with the 6° flight path is 1980 m or 6,500 ft. A more precise solution would include the fact that the lift requirement goes down with the cosine of the flight path angle, but for a first-order estimation the numbers here are good enough.
This is not exactly treating the airplane as the handbook would suggest it, but the type has survived worse treatment before.


text:
How to stabilize the aircraft flight after dropping boosters?
I want to make aircraft model with boosters, but i don't know how to stabilize it. Аfter dropping the boosters, the center of mass of the aircraft will change, and I would also like to know whether it is necessary to stabilize it at all? Also during the flight on its own engine with fuel consumption, its mass will also change, whether in this case it is necessary to stabilize it? I need methods/ways or sources of info about it (maybe books, articles). Thanks for your attention.

Welcome to the site @AiriPad. Your question isn't very clear, what boosters are you talking about? Why would they drop? The question is also broad, covering aerodynamics, stability, weight distribution, and more. Those topics could fill a library, you need to make your question more specific.

I'm voting to close this question as off-topic because it is about designing model aircraft and is unrelated to actual aircraft (and is also an incredibly over-broad).

The most important aspect is the vertical distance between the booster's thrust vector and the center of gravity. Once the booster stops working or is dropped, a pitch trim change will be needed to compensate for the upset in the pitching moment balance.
Next, the horizontal distance between the boosters and the center of gravity will tell you how much the pitching moment is changed upon dropping the boosters.
If control authority is sufficient to trim all those pitching moment changes, stabilization should not be much of an issue, especially if a sufficiently sized tail gives sufficient pitch damping.

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text:
What is the shed bus in the Phenom 100?
I recently came across a part of the electrical system of aircraft named SHED BUS. I am specifically asking for the Embraer Phenom 100.

Source (Page 122), Added red square

What is the SHED BUS for and what is it doing?

The shed bus powers non-essential aircraft systems. In the case of the phenom 100 this is things like the air conditioning, passenger power sockets and entertainment systems, the toilet and some lighting.
The shed bus is the first thing to lose power (be shed) should the aircraft not be generating enough electricity through, for example, a generator dropping offline.

A SHED BUS is used to provide power to non-essential systems.  By powering off the shed bus, all non-essential loads can be removed in the case, for example, of a generator or engine failure with a single action rather than having to switch systems off one by one.  This might happen either automatically or via a cockpit switch or both.

On the Phenom 100 & 300, the Generator Control Units (one left and one right) control the contactors and will automatically disconnect the Shed Bus Contactor under certain conditions. I don't remember if there was an external control as well.

Embraer aircraft have automated Electrical Distribution Logic (EDL).  The intent to to minimize crew workload. Part of the EDL protects the generators from being overloaded. It does that by disconnecting the Bus Tie Connector (BTC) for the Shed bus. This typically occurs when one generator (or more) goes off line.
The Shed bus itself provides power to non-essential systems, such as the galley and reading lights.

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text:
Why has the A340 not been fitted with only three engines?
The A340 was created because the airlines wanted an aircraft they could cross the atlantic ocean with (They couldn't with their twins because of ETOPS so far). So Airbus upgraded the A330 and added two engines. Because of the four engines, the plane was now allowed to cross the ocean.
However, four engines instead of two increase fuel cost dramatically (I assume). And the A340 is the same size as the A330. The A330 could fly with 2 engines. So why hasn't Airbus built the A340 with three engines instead of four. The fuel cost could have been reduced significantly without loosing the permission to cross the atlantic ocean. For example the MD-11 had three engines and also had permission to cross the ocean.

Why should adding engines cause a dramatic increase in fuel use, given same total power output, cruising speed, &c?

For me, as naiv non-engineer, more engines mean more fuel-burn :) But yes, I get your point.

@NoahKrasser: not aviation related, but a few years ago I changed my van with a 4.3 liter engine for a newer van with a 3.3 liter engine. To my surprise, fuel consumption was indistinguishable. I assume that if the engine doesn't use a significant amount of fuel to move itself, the absolute majority of the fuel burn is used for propulsion, and in conclusion the number of engines shouldn't make a big difference in fuel burn.

I think the premise of the question is entirely wrong... wasn't the A-330 always an ETOPS-approved aircraft? With the reason for the 4-engine A-340 being ability to carry more (fuel) weight in order to get longer range?

From what I have understood: Because engines weren't very reliable at the times of A330 and co., it was prohibited for twins to cross the ocean (They always had to be in range of a airport they can divert in case of an emergency).

Because the third engine would need to be near on centerline. That means in the tail.
Putting the engine in the tail significantly changes its design requirements.
Supporting an engine requires a lot of structural support. Then you need to run fuel to it. The effective height of the rudder is decreased. You probably need to switch over to a T-tail. The center of mass is also shifted which influences where the wings need to go.
In short, adding a single engine requires a redesign of the entire tail. While adding an engine to the wings is much simpler.

Agreed for the most part, but why switch to t-tail? Neither DC-10/MD-11 nor L-1011 were t-tail. The h-stabs were mounted well beneath the #2 engine in both of those cases.

@reirab that's why I said probably, it'll depend on how the main supporting structure is inside the tail and whether the new engine will get in the way

The placement of a tail mounted engine would produce a large change in the center of gravity. This would require a redesign to "move" the wings rearward, ie. add fuselage sections in front of the wing and/or remove sections from behind the wing. In addition going from 4 to 3 engines would require a total re-design of the fuel, electrical and hydraulic systems. All these changes mean that you have an entirely different aircraft which would have to be re-certificated from scratch. They would be better off with a blank sheet of paper.

Adding engines to the wings allows the minimum amount of changes to be made to the aircraft. Adding a third engine would require re-designing the tail section, and shifting the CG considerably at the same time. That would affect the airplane's handling characteristics so much that many calculations have to be redone.
In fact, the A330 and A340 are so similar that pilots are allowed to operate both of them with very little additional training. For example, Cathay Pacific pilots who are trained on the Airbus can operate both A330 and A340 in their fleet. This adds flexibility to an airline's schedule and reduce their training cost. If the A340 were a tri-jet, that would not be possible.

Project decisions like this based on are return on investment for the manufacturer.  In other words, what will sales revenue be compared costs to engineer, certify, manufacture, and support be compared to sales revenue for the life of the project?
A center engine would require redesign of the empennage and all related systems.  The certification requirements and cost to add an engine to the fuselage would be astronomically higher than hanging additional engines on the wing.  Additional engine nacelles would have many parts and assemblies in common and would not impact as many other systems, flight controls, and aerodynamics to name a few.

It only needs 2 engines.
Twin engine jets have had ETOPS ratings for decades, 757, 767, 737, etc. The engines on modern planes are gigantic, 8 ft dia fans and provide 50,000 to 60,000 lbs thrust, so they are plenty powerful.
Tri jet designs such as L1011 (beautiful design) and the DC10 had the issue of difficult maintenance of the high tail mounted engine and as technology improved, that configuration was phased out, although many tri's are still in service.
Quad engine designs are disappearing, too, except for heavy lifters and low wing designs that would cause clearance issues if larger twin engines were used.
No reason to use more engines than needed for the purpose.

This is totally true now, but it specifically was not true when the A340 entered the market.

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text:
Are pilots flying parabolic flights less prone to sickness than their passengers?
Parabolic flights to simulate zero g are notorious for inducing motion sickness in their passengers. Are pilots equally likely to get sick during such flights? If they aren't, why not?

They are less likely than occasional passengers to become sick for several reasons:

They do it more frequently and therefore become conditioned to the feeling of weightlessness.
They can see the horizon and know what to expect, so the primary cause of motion sickness, (a mismatch in the body's sensory inputs) is mitigated.
People who are very sensitive to motion sickness generally don't seek professional pilot careers, especially in such a specialized field as this, so there is a natural screening process in place.

There may be other contributing factors as well, but these are the main ones.

I'd imagine that prior to becoming a vomit comet pilot, one takes a ride in the back of said vomit comet. If one vomits, one fails the interview process and goes on to a different piloting path.

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text:
Do Blended Wing Bodies produce more induced drag compared to a conventional tube and wing design? Does this affect its performance?
Blended wing bodies can produce more lift by its aerofoil configuration. It has been shown to produce up to a 30% decrease in fuel burn due to its additional lift, compared to a tube and wing plane.
My question is: does this extra lift produce more induced drag? Could this induced drag cancel out the aerodynamic advantage of the lifting fuselage?

Can you add some links about that claim?

They don't produce additional lift.  Aircraft produce lift equal to weight.
The added lift of a fuselage may reduce the angle of attack required by the wing to achieve lift equal to weight -- but for aircraft in cruise, there is no such thing as 'more lift'.
All lift contributes to induced drag.  The induced drag can be determined by looking at the downwash of the wake after the aircraft passes -- it does not care whether that lift came from wing, body, tail, etc.
For induced drag, what matters is

total lift (which equals weight)
span that the lift was created over
the shape of that lift distribution.

So, the best way to reduce induced drag is to reduce weight.
The second best way to reduce induced drag is to increase span.
The third best way to reduce induced drag is to improve the shape of the lift distribution.
Tube & wing aircraft will show some lift deficit at the centerline because the body does not contribute significant lift.  Adding some lift to the body can fill that in and consequently improve the lift distribution and thereby reduce induced drag.

Thank you. Would it be correct to also say that a blended wing body increases the lift-to-drag ratio compared to a tube and wing, rather than producing "additional lift"?

Done right, that is certainly the hope -- but it is likely that the improvement comes from reduced wetted area (and attendant skin friction drag) by eliminating the fuselage and tail.


text:
How does engine exhaust temperature affect contrail formation and why?
There was a german study about the "Influence of propulsion efficiency on contrail formation". In this study, an Airbus A340 and a Boeing 707 flew right next to each other.

The two contrail forming aircraft were
(i) a Boeing B707 equipped with four jet engines of type JT3D-3B with bypass-ratio of 1.4 and
(ii) an Airbus A340-300 with four jet engines of type CFM56-5C4 with bypass-ratio of 6.8.

The A340, with the high bypass-ratio, formed contrails behind it, whilst the Boeing with the low bypass-ratio did not form any contrails behind.

Source
This has to do with the different exhaust gas temperatures of engines with different bypass-ratios.

How exactly is the formation of the contrails affected by the EGT (and therefore bypass-ratio), so whats the difference between low EGT and high EGT in terms of contrail formation? What is the physics behind it? Just why does the Airbus form a contrail, whilst the Boeing doesn't?

Note, 'How does contrail formation differ from turbofan to turbojet?' does not answer the question regarding the EGT.

Seems to be a duplicate of How does contrail formation differ from turbofan to turbojet? with an answer showing this same picture.

I think you have it reversed, high-bypass ratio engines have lower exhaust temps than low bypass-ratio engines because more air is bypassing the actual combustion in high-bypass ratio engines.
tldr; Temperature and pressure are directly related; if the air is hot the amount of water it can hold decreases. The water molecules from the hot low-bypass exhaust move faster to lower pressure (ambient) air surrounding it causing a lower humidity meaning less or no contrail while the water in the colder high-bypass hang around longer

@NoahWood This already sounds like a good start for an answer.

Note that the linked answer does not talk about EGT, because the linked study does not directly either. Because what matters is the remaining energy, i.e. how far the water is from condensing. Higher temperature generally means more energy, but it's not the only factor.

@mins
No I'm pretty sure I said what I meant. Clouds are suspended water droplets. Hot air will evaporate the previously condensed water droplets and raise the pressure causing the water vapor to move towards a lower pressure where it will re-condense into water droplets and ice crystals.
Perhaps 'amount of water it can hold' was a bad phrase though I can see why that might be confusing. Assuming an equalized pressure at all times in two separate containers of equal humidity, the heated container of air will flow out into the general atmosphere while the chilled container will condense.

There are some building-blocks to the answer:
Part 1: Assumption/Simplification
Let's assume for a moment that the only difference between the two planes is the exhaust gas temperature (EGT). A less efficient engine will utilise less of the available (chemical) energy of the fuel. This means a less efficient engine will have a hotter and faster exhaust jet (wasting caloric and kinetic energy).
Part 2:
The water-content of the exhaust gas contains water from two sources: first the water (very low fraction) from the ambient air humidity and second from the combustion of the fuel (carbon-hydrates).
Part 3: Assumption
When humid air (from an engine exhaust) mixes with the atmosphere it is a reasonable assumption that the state of the atmosphere will not change measurably given enough time for the exhaust to mix out.
Part 4:
The mass of water vapour which can be contained in a given mass of air depends on pressure and temperature of the air. This dependency is often visualized in either the Mollier-Diagram or Psychometric-Chart. The following simplification of a Mollier-Diagram shows two areas or states humid air can have. For a given temperature and low water content the water is in its gaseous phase. If the water content is increased (over the saturation point) than part of the water will be condensed (fog, small water droplets).

Part 5: Answer-Part I
When hot humid air exits the engine it will cool down an partially/continuously mix with the atmosphere. This means temperature and water content/ratio (per a given volume) will decrease. The following sketch illustrates this for two starting points. At the end of this process the jet reaches the state of the atmosphere (see part 3). In the case of a lower EGT the cooling of the exhaust can cause the humid air to cross the saturation-line which will result in a contrail.

Part 6: Answer-PartII
A lower by-pass-ratio-engine is likely to have a higher velocity difference between the ambience/atmosphere and the jet from the engine. This will increase the mixing of the exhaust gases which suppresses the condensation as well.
The model used in the paper (Schumann 2000) also includes basic thermodynamic considerations for the jet-engine which do increase the predictive accuracy of the model. But the thermodynamic effect should be captured by the parts 1-6 given here.

I would guess that the Fig.3 in the paper is a psychometric plot, which is a flipped Mollier-Diagram (used in part 4, 5).


text:
Is it possible to install a bike rack on a Cessna 172?
I stumbled upon a nice photo manipulation on aviationhumor.net.

Source
My question is short and concise:

Is it practically possible to do that? Is it okay in terms of aerodynamics, weight, CG?
Bonus Question: If it was possible, would there be any chance it gets certified for use?

You might have more luck hanging them under the wing

That would be a pretty bad day if one of them lets go & takes out your rudder... and even if it didn't damage your aircraft, it could ruin somebody's day on the ground.  I don't really see the FAA agreeing to something like that, since every single time you fly with bikes, it's a new installation (of the bike on the rack) and the need for checks that nothing will let go at an inopportune moment.

@Pondlife Hmm.... I don't think that's what "bicycle landing gear" is supposed to mean.

The wing is pretty much designed to be at the center of mass of the vehicle, so putting something above the wing is actually the best place for it.

Not really an answer so a comment.  I had a similar set up on 120 hp Civic and it was a lot of drag.  I took almost 20 mph off my top speed.

Perhaps you should make it clear whether you are asking whether such a bike rack is practical out of purely academic interest, or whether you actually want to carry bikes in your 172?

@jamesqf Only academic interest :)

@jamesqf what's unclear about the current phrasing? To me it seems quite obvious that the interest is academic and it is all about having a bike rack, as per title.

What if you laid the bikes down, and had some sort of cover to go over them? Kind of like a car roof top cargo carrier, but for bikes?  Or put them on the side of the plane with an aerodynamic cover?

Is it practically possible to do that? Is it okay in terms of
  weight, CG?

Bikes (depending on make and model) are not all that heavy and in this picture they are more or less center mass and would most likely fall in the CG range.

aerodynamics

Again this depends on the bikes and the placement but with proper care you could be ok. Admittedly a bit different these bikes would create no more drag than say a wing walker would and people have been doing that for some time,

(source)
keep in mind bi-planes fly a bit slower than a 172. But on the small vehicle transit front the PBY-Catalina carried a full sized rowboat under its right wing for use when you arrived at your destination. Interestingly it seems to be mounted backwards which I would think is the non aerodynamic way to mount it...

(source)
It looks like at least one guy has actually managed to pull this off in New Zealand

(source)

In case you were wondering if it’s legal, the Civil Aviation Authority
  of New Zealand has approved the design.

And he used a Cessna 185. Important to note that these are mounted under the wing which keeps them out of the way of the rudder as Dan mentions but more importantly should anything go wrong the bike would shear away and fall down away from the aircraft instead of back into the rudder.
It looks like this kit plane also offers a similar rack.

....If it was possible, would there be any chance it gets certified for use?

In New Zealand yes, in the US under the FAA, depends how good you are at paperwork.

"these are mounted under the wing which keeps them out of the way of the rudder" ...but your horizontal stabilizer is probably toast if the bicycles somehow separate from the wing in flight. (It doesn't look like they'd have time to drop sufficiently in cruise to clear below the tail, and they definitely aren't going to clear above the tail.) I'm not sure I'd prefer either to the other. Something like a snapped elevator cable is bad enough; I feel that a bicycle (or two!) hitting the front end of the stabilizer at 150+ km/h likely wouldn't be better in terms of resulting aerodynamics.

@MichaelKjörling I'm a physicist not a pilot, but if those bikes came off they wouldn't hit the stabiliser at anything like you airspeed.  They'd start at the airspeed and decelerate due to their drag.  If the bike decelerated instantly on detachment it would fall about 4cm before the stabilizer hit it (@95kt and scaling off a photo).  At the same speed the drag force on a bike with rider would be ~420N, which would decelerate a 10kg bike by 42m/s².  The bike would hit the stabilizer about 0.4s after detachment doing 30kt (actually much less as no rider). It would fall 30cm in that time

Still too much for comfort.  BTW I couldn't find figures for the drag on a bike without rider. The forces aren't very much higher than on top of a car (you should see how fast some people drive with roof loads round here)

Have you got a contact phone number for that NZ guy?  I need to install a hot-tub on top of my Chinook.

@MichaelKjörling The points where struts meet the wing on Cessna 185 look like about same distance from the centerline as tips of horizontal stabilizer. Racks seem to be hanging further outside, which gives you slight chance that a falling bike will miss the stabilizer horizontally as well. Or clip only the outer half of the stabilizer.

@Agent_L Good point. I'm still not at all sure I'd feel comfortable with it, though...

When using the bikes on the last one, pedal clips are recommended.

@Sneftel Heck, hook up a dynamo and you can call it a "green" aircraft! Helmet recommended during takeoff and landing.

Related question at bicycles.se.  The drag on the bike (assuming it kept the same orientation, which it wouldn't)  would be <1/10 of my estimate with a rider, meaning that after detachment it would take over a second for the stabilizer to catch up and have a closing speed over under 10kt.  But in this time it would have fallen clear of the plane by a few metres.  This is the other extreme, as once the bike started tumbling it would present much more drag surface

@Wossname it looked that way to me too, but the bikes wouldn't keep pointing that way if they came detached, and may even turn side on.

Off topic but, I googled the tail number of that plane.  Freaky!  I did not put in any other text except the tail number.  The "images" tab returned about sixteen photos of that same plane, plus photos of other similar looking planes, plus one photo of a different looking plane, on the ground, which appeared to have an empty bike rack fixed to the underside of its wing.

@ChrisH: I think you'd only be in trouble if the manoeuvre that shook the bike loose from a failing rack was the start of a dive (so you fly the tail into the falling bike).  Then it could maybe get caught with the seat holding it onto the horizontal stabilizer and do some damage until it shook off?  (If tumbling). This sounds highly unlikely to happen by accident, though, and the tail is far behind the CG so it actually moves upwards at the start of a dive, but I think the diving aircraft would bring the tail with it down below its starting elevation very quickly.

@PeterCordes my calculations suggest that's the case, but only because the bike is moving forwards at the plane's speed to start with. It would be rather like dropping an under wing bomb, though less streamlined. If the front wheel detached first, the situation may change because the bike would pivot about the back wheel

These New Zealanders https://www.helibikenz.com/ have a business flying bikes up mountains, albeit on a helicopter.

Just eyeballing your image, the CG looks OK, and it's possible to put that much weight into the interior of the plane just fine, so the weight is OK.
The aerodynamics on the other hand - yeah, no.  Imagine the bikes are not strapped down and you have to hold them in place - in a 120 MPH wind.  That's a LOT of drag and the turbulence from it could easily mask the rudder.
As for certification - in theory you could get an STC for all Cessna 172s or file a form 337 for a modification for a single named aircraft.  You would have to prove that this was safe and the aircraft was airworthy.
Good luck with that.
It would be far cheaper/easier/safer to just buy a folding bicycle and carry it inside the cockpit.

Don't know about a 172 specifically, but a mountain bike (plus camping gear) is easily carried in a Piper Cherokee.  You just remove the upright part of the rear seat. (I eventually also removed the base, and replaced it with a plywood sheet cut to fit.)  Then pop the wheels off the bike, and it fits in fairly easily.  Use a cargo net to keep things from bouncing around.  I'm pretty sure two bikes would be doable.

If you take the question literally, then it's probably possible to install the rack, as long as you never, ever put bikes in it. Except when the plane is parked.

@dreamcatcher Very good point. I think it's possible to go even further and get approval for transporting bikes while taxiing at very slow speeds.

A folding bicycle is not going to substitute for a mountain bike.  Still, overall I like you answer.

@dreamcatcher That'd be a way to get a lot of attention at Oshkosh.

@Paparazzi's right but you could always build a frame with S&S couplers


text:
Why would the F-16 pilots chasing Flight 93 on 9/11 have needed to ram the airliner?
I realize that this is an old topic, and some elements seem to have been answered in the question below.  However, one of the two F-16 pilots who scrambled to intercept United 93 just retired from the USAF, so it’s back in the news.
Both pilots said that they planned on a suicide mission that would have involved ramming the airliner.  Some responses are here:
< https://aviation.stackexchange.com/questions/80150/the-f-16s-that-were-scrambled-to-intercept-united-93-did-not-have-time-to-arm-th?newreg=c4c0dc4dad354f98938379776d829176>
My questions:

Couldn’t they have likely disrupted the plane enough to make it crash by using a sonic boom right over the cockpit?  My colleague, who flew F-16s, aaid that the pressure wave from even a few feet was crazily intense — enough to flip cars over if it were a low pass over a street.  If not over the cockpit, are the wings or rudder designed to withstand that level of pressure?
Couldn’t they have simply settled the two jets directly in front of the airliner or its engines, forcing the airliner to fly in their jet wash?  I gather that the Top Gun jet wash phenomenon is real.  Is the issue that commercial airliners are so stable that all the jet wash would have done is to produce turbulence?  Seems like they could have gotten close enough that there wasn’t even enough oxygen entering the engines to keep them firing.

"flip cars over"  That would be the thrust, not the "pressure wave."  For the F-16 to apply its full thrust to the airliner, it would need to ram the airliner.

A small GA plane? Yes, a pair of 20-ton jets can damage a <1 ton GA plane without contact, guns, or missiles.
Sonic boom over the cockpit will do little to disrupt the flight of a 100-ton part 121 airliner, except startling the pilots for a second. Pretty much everything about an airliner is built to survive wind gusts, turbulence, wind shear, and other aerodynamic phenomena.
Nor will the jet wash shut down commercial engines running normally in level flight - they are notoriously difficult to shut down even with water. Jet engines only deplete the air from 21% oxygen to about 18%-20%, depending on their bypass ratio, which is well enough to sustain combustion.
The passengers would feel some turbulence, and an engine might choke momentarily, all of which would be bad PR for Al Qaeda Air, but it doesn't appear to have had any plans for a frequent flyer program in the first place.

+1 for facts and entertaining writing.


text:
Why don't small props have a gear?
Small GA-Aircraft like a Cessna 172 or a Piper PA-28 have an engine similar (I think) to the one of a car.
But why don't planes have a gear and a lever to change the gear?

Would'nt they achieve a faster prop speed by doing that?
Would it be possible?
Would it be useful?

Because there is no link between the wheels and the engine.
When the car is moving slowly, wheels turn slowly so the engine must turn slowly as well. However piston engines do not work below certain RPM, so the gear switching must be implemented even for small cars.
Propeller can always turn at the speed sufficient to sustain the engine running. If the plane moves too fast (or must stand in place), wheel brakes can be applied.

"When the car is moving slowly, wheels turn slowly so the engine must turn slowly as well." Sorry, but that's a completely inaccurate analogy of how an engine/transmission works. When the car is stopped is the engine off? You can place your foot on the brake in a car as well, car stops, engine keeps running. In many cars there is no direct link between the engine and the wheels either, the torque converter is in the way (which uses fluid to couple them together).

Ae, stop the car by placing foot on the brake and watch your engine still running ...

The question is about the difference between cars and planes. Most often both have wheels.


text:
Can I buy a A320 FCOM, FCTM and QRG as non-pilot?
I am very interested in the Airbus Systems. I really like the FCOM of the A320, there are some of them on the internet.
However, I like holding physical books in my hands more, than reading them on a screen. But I don't have the paper nor the print color to print more than 1000 pages.
I would really like to buy a FCOM of the Airbus A320-Family. Can non-pilots, like me, buy a FCOM? If yes, where?

Not an answer to your question, but can't you just let your local print shop print and bind it for you?

@Sanchises I calculated it. It would cost 40 € to s/w print the FCOM. Maybe I can get it cheaper.

Perhaps contact Airbus?

What Kevin said. What have you tried to do in order to answer your question yourself before posing it here? At least some of that information really should go into the question.

I'd guess that a local print shop cost of  40 € is probably going to be less than getting an official copy from Airbus. But that's just a guess... Have you asked them?

I would second @FreeMan suggestion, take the PDF to a print shop (like FedEx kinko's or any number of online services) and see what it costs to have one print and bound. They may have to bind it in volumes for you, but it should be much cheaper than trying to buy one from Airbus.

I thought about a website selling (used) FCOMs or something similar. Good idea to contact Airbus, I will do that.

@NoahKrasser I actually own 3 sets of airbus 320 FCOMS original prints with airbus binders and all, I also have soft copies. The reason it’s very difficult to find is that airbus update their manuals frequently and hard copies are kind of obsolete now...but if ur interested we can probably arrange something.

@OverD Thanks for your kind offer, but I wrote this question when I still was in school and I had more free time. I still love aviation, but don't have the time anymore. Thanks though.

@OverD do you still have original A320 FCOM? Old version with 4 volumes or new version with 6 volumes?

@Afe I have the old original FCOMS along with QRH that was given to me by my previous airline

What is the condition? Can we chat privately?

@Afe they are in great condition, they have not been updated since 2005. But the binders and all are all original


text:
What temperature are Pitot Tubes heated to?
What temperature is a pitot tube usually heated to prevent ice?
And are pitot tubes always heated to the same temperature, is there any difference between aircraft (say, a C172 and an A321)?

According to this article: http://www.matronics.com/aeroelectric/articles/Pitot_Heater/Gauging_Pitot_Heater_Performance.pdf - the internal temperature of the pitot-tube was around 270degrees celcius while the unit was placed in submerged water. This was while operating at 13V and 16A (208W). This is the type of pitot featured in a light aircraft and is most likely similar to those fitted onto C172s.

@andy-m Why don't you post an answer?

@NoahFisher Because it doesn't answer the question. The question isn't "What temperature are pitot tubes heated to?" but "Are they all the same temperature?" A single datapoint is an OK answer to the first question, but doesn't address the second at all.

@DavidRicherby Actually the first was exactly my question until Jamiec changed it to what it's now.

@NoahFisher OK but the comment was posted after that edit. Apologies, though, for telling you what your own question said after it had stopped being your question. I'd not noticed you were the asker or looked at the edit history, and I should have done both.

@NoahFisher really? You original question was On what temperature is pitot heat heating the Pitot Tube and are there any differences between, lets say, a C172 and an A321? Thats still 2 questions of which only the first is answered by that comment! (I think you meant, "thank you for turning my garbled English into something resembling actual English")

@Jamiec Yes, thank you a lot for turning my english into readable text. But you deleted one of my two questions from the main body of the question. It was still in the header, but I think it's good SE-practice to write it into the body again.

This page talks about the Pitot probes that operate at temperature range -65°C to +70°C. They are FAA TSO-C16 and Military Approved. It is probably unwise to heat them more during normal operation. This is not very hot but +70°C would already burn the skin. The probes are used at Pilatus PC-12, Bell 212, Bell UH-1 Special, Cessna Caravan, Diamond DA42, IAI Heron and Quest Kodiak.
This contradicts another source where heater temperatures of 200 °C are reported, and also this industrial Pitot device (while not used on the aircraft) is rated up till the 200 °C, so the design itself allows the temperatures that high. However the heating device is only part of the probe and should be hotter than the tube it is heating up.
This answer says there are significant differences between Pitot heat systems, and some on the light aircraft are rather "anemic". Hence it may be that some models are heated and operate at much higher temperatures. There is also a phrase "plastic cover could melt" there. Plastic melts well over 150 °C.

In which conditions is the temperature measured? And what is the thermal inertia?

This is not specified, I assume these tubes are not rated above +70°C in any possible conditions.


text:
How to read the Appleman chart to predict contrail formation?
I stumbled upon the Appleman-chart, that makes it possible to predict the formation of contrails.
How do I use this chart?

A well researched question.

Very nice! Thank you for sharing this useful procedure and the sources used. Something more on my agenda -- a contrail predictor program or map.

@not2qubit Yeah, it's an incredibly interesting topic. But just be aware of one thing: During my further research, it came out, that the Appleman-chart is made for low-bypass-engines. Jeffrey Peters has made new charts for high-bypass-engines. Here is his work. The new charts begin on page 20 (Chrome), respectively page 14 of the document.

Please copy this into an answer as ymb1 suggests. It really is good and needs no further answer, but keeps popping to the top of the list of unanswered questions.

@Pilothead Didn't know this happened, I moved it.

First of all, the Appleman-chart itself:

Source
Now, let's say we want to know if contrails can form at FL340 at my position, a pretty standard cruise flight level. Because flight levels are standard pressure, we can easily look up the pressure, corresponding to the flight level.
100 ............................................ F530
150 ............................................ F450
200 ............................................ F390
250 ............................................ F340
300 ............................................ F300
400 ............................................ F240
500 ............................................ F180
600 ............................................ F140
700 ............................................ F100
850 ............................................ F050

Source
Ok, we've got the pressure, it's 250 hPa.
Now we need the temperature at this flight level. I am using a winds aloft chart for this.

Source
Let's just take a value from what's nearest to my position, -56 degree Celsius in this case.
Now we can look this up on the chart.

Ok, looks like we've got contrails to see. But am I missing something?
I quickly tried to check the result, by looking out of the window. I expected following:

Contrail existing
Very short contrail. I looked up relative humidity for that flight level as well. It's very low, only 3 to 1 percent. And the lower, the humidity, the faster a contrail sublimes and fades away.

Here's the data for the humidity:
-------------------------------------------
PRES   HGHT   TEMP   DWPT   RELH   MIXR
hPa     m      C      C      %    g/kg
-------------------------------------------
258.0   9699  -54.9  -79.3    3   0.00
250.0   9900  -54.3  -80.3    3   0.00
222.0  10665  -52.1  -82.1    2   0.00
200.0  11340  -52.9  -83.9    1   0.00
198.0  11404  -53.0  -84.0    1   0.00
185.0  11839  -53.4  -84.4    1   0.00
150.0  13180  -54.7  -85.7    1   0.00

Source
So, I first looked on flightradar to search a plane I could possibly spot from my position.

Picture from flightradar24.com
UAE72 is at FL390, what is a standard pressure level of 200, so we've got a relative humidity of 1%.
Now the look out of the window:

As we can see, it's exactly as expected.


text:
Why does Airbus not display the exact airspeed on the PFD?
On every other PFD (I know), the exact airspeed is displayed as big number on the PFD. To clarify what I mean, here are two pictures:

Source, added square, Boeing PFD

Source, added square, D10A EFIS
Now, when we take a look at the Airbus PFD, we can see that there is no exact speed display:

Source, added square, Airbus PFD
This is one of the biggest differences between the Airbus PFD and other PFDs. What led Airbus to this unusual decision? Is it maybe because Airbus pilots don't need to watch airspeed as close as other pilots, because of the numerous flight envelope protection systems and the fact that the autothrottle controls the speed until 20 feet above ground?
Is there just no need to know the exact airspeed and Airbus wanted to remove a source of distraction?
Is it possible to turn an exact speed display on if the pilot wants or needs it?

14 up-votes, an open bounty, and no answer. I'd put it this way...... no body knows why Airbus did that......

@kevin To be different from Boeing? No seriously... with all the odd peculiarities in Airbus design (additive control inputs, non-mechanically connected flight controls, two completely different feeling controls - left and right stick, strange PTU noises that terrify passengers, and now a non-standard PFD layout), I'm starting to think they more and more simply wanted to be not like Boeing.

I think it's a pretty good decision to put the exact speed away. Because they must have thought something when they did it and it's definitely a distraction less. But I don't know why they did it, that's so interesting.

By exact airspeed are you talking about the indicated airspeed (or possibly the calibrated airspeed) or the true airspeed? If you're talking about true airspeed, I would suggest that they may have decided to not display it because it's not a useful value for the pilots. The mach number is useful at high altitude and indicated (or again calibrated airspeed) at low altitudes, but the true airspeed is only a curiosity, at least in my experience. It was in no way displayed on our old steam gauges on the 747-100/200.

@Terry I am talking about the indicated airspeed, just the exact value that is displayed on the speed strip. On the airbus PFD one can only read the approximate value.

@Terry I find our TAS display to be useful since it has value in comparing actual performance to flight planned performance. Our TAS is a small window separate from the IAS display.

@JonathanWalters That certainly seems reasonable as a quick reference. Back in the dark ages of the 747-100/200 we compared actual performance to planned performance by keeping track of the time of arrival over each flight planned point to the projected time and, more importantly, to the actual amount of fuel left compared to what was supposed to be left. This was all done on paper flight plans, a copy of which both the two pilots and the flight engineer had. One copy went after each flight to company headquarters for possible review by management and/or the FAA.

I always thought the yellow speed line (vs digital in a box) is a way of indicating the speed (and speed trend) are aproximations and subject to change ( ie. indicating a speed of 134.4345 kts implies a very high precision, which the ADR does not have)
In reality (after taking a long time to get used to it) I started to preffer the Airbus method: the yellow line allows my eyes to focus on the entire speed tape, including the speed trend vector and the upcomming speed limitations on the bar, giving me a better overal picture of the speed.  The old (Boeing?) display would capture my eyes on the exact number, to the detriment of the other indications on the speedtape. This is particulary true on turbulent approaches when speed fluctuates a lot.

Yes, you are so right. I love EFIS, but I think for speed display, the old steam gauges are better. Whenever I am in using an EFIS, I always automatically look at the speed standby gauge.

Disclaimer: this is more a beautified comment than a real answer as it doesn't really answer the question. Feel free to ignore, or even downvote if you feel punitive.

Although a nice question (and indeed a good observation) this is a dangerous one: it involves guesswork. As @kevin already mentioned, who knows why they made that choice and why?
This involves UI and UX and it's a subjective(?) matter by nature. Some History: Many years ago, Air Inter Flight 148 crash happened involving a A320. One of the reasons that contributed to the crash was the UI/UX: Flight Path Angle autopilot mode display was very similar to Vertical Speed mode. Only one dot difference. See this FAA article for the details  but the image (taken from inside the article) depicts the concept:

That particular design choice was proven terrible after all. But why did Airbus make it, especially if it was dangerous? We don't know and probably never will, and my guess is they didn't think "let's mesh things up and see what happens".
So now back to your question, maybe they concluded that adding it was cluttering the airspeed strip. Maybe they "asked 100 pilots" looking for the most popular answer to the question: "Do you want to see the numeric value on the airspeed strip?" Maybe it was overlapping with the acceleration-deceleration arrows and other indications on the right side of the airspeed strip, drawing the pilots' attentions away of the bigger picture as Radu94 already mentioned in their answer.
But here is the problem, all these are maybe. Hypothesis.
If you feel bold enough, I would suggest to contact Airbus and directly ask them. You might be lucky enough and get an answer (and be kind enough and share it here :) ). I got one from EASA regarding ATC simulator regulations. Although that might not be the same, since if they have put really big effort and manpower in researching and designing this, they might not want to share what they will consider company secrets.

Also, maybe they kept it more like a traditional "steam" gauge which doesn't show the exact number either.  ;-)

@Lnafziger Indeed, good thought! I was thinking of adding it in my answer but then the list just grows bigger without any certain answer but only speculations. :)

I am UX designer with background in information architecture and thought i would contribute
I don't have a definitive answer but just an informed guess.
In a sense all of the answers raise valid points.Considering the context; in environments where user perform complex task the design of the interface has among other objectives the need to reduce cognitive load. As the brain has limited amount of processing power, too much information delivered in too short time particularly where a lot of other elements are competing for attention can increase cognitive load to a point where it could be overwhelming. I think, variations when displaying airspeed format could be a symptom of that.
Interface designers can reduce this cognitive load by reducing extraneous visual elements such as in the case of the airspeed display for airbus.
This is where it gets a bit tricky because based on my understanding airbus and Boeing have different philosophies in terms of handing control to the user/pilot which include displaying system status or what we refer to in UX heuristics as:
"visibility of system status"

The system should always keep users informed about what is going on, through appropriate feedback within reasonable time.

The heuristics also include guidance which promotes :
Recognition rather than recall

Minimize the user's memory load by making objects, actions, and options visible. The user should not have to remember information from one part of the dialogue to another. Instructions for use of the system should be visible or easily retrievable whenever appropriate.

So, overall i think we have two competing heuristics in this particular case and choice is largely based and each airline approach.

Welcome to Aviation.se, thank you, hope to see you around!

In a sense, the Airbus does display the exact speed on the PFD. It just requires the pilot to interpolate between the markings.
If you need to the know the FMGC Managed target speed/Mach, it's available from the PERF page on the MCDU while a Selected speed/Mach is shown in the speed window on the FCU.
In almost all case we select a speed in multiple of 5s while the FMGC managed speeds usually (but not always) flies a window of speeds around a desired value depending on the current mode and configuration.

A breakdown in the Usability Testing process?
In software engineering, user facing applications are often put through a second stage of testing called usability testing.  This differs from functional testing, which is designed to root out bugs.  With usability testing, application flow, ease of use and intuitiveness of the UI are tested.  Companies that can afford to do so will often bring in panels of real users to find out how they perceive an applications usability.  As a software engineer and someone who manages software engineers, I've sat in on a number of these testing sessions over the years, getting first hand feedback on the products my groups have produced.
I would have to assume, for such high dollar item, that Airbus would put the PFD through a fairly rigorous UT phase, certainly with real pilots.  But no process is perfect, and it's just possible this was missed.  This would also mean it was missed in the requirements gathering phase of the project as well, which is really a surprise given how standard this type of display has become.
Of course it is also possible that the usability testing concluded that the exact airspeed should not be put on the PFD.

remember the A320 was designed in the 70s/80s when Human-Machine interfaces and UI design was not so evolved. I wonder if any usability testing/design ever went into the display units beyond applying CS-25 conventions

Ah yes, the CS-25 conventions...just kidding, had to look that one up ;-)  Agree though, the testing conventions I speak of certainly would predate that.


text:
What distinguishes the Airbus-Sidestick from an ordinary gaming joystick?
I don't think the Sidestick in Airbus aircraft is an ordinary gaming joystick that is plugged via USB into the Airbus Computers. I guess it is something very special, because it's one of the most important and critical parts of the aircraft.

What are the differences between an ordinary gaming joystick and an Airbus Sidestick?

Related on our friend Quora: How does the control joystick in Airbus aircraft work? and beautiful images too at AirCockpit. Is your ordinary joystick like this?

You should consider making this into multiple questions. (cost, mechanism, etc.). You are basically asking four questions here.

@JimyPP Sounds good.

Much more reliable. My Saitek X52 broke just 3 years after I bought it, I had to screw it open and solder broken wires to get it working again.

The differences (1)  is what's happening when the game over screen comes up; (2) the context of what "crash" means when using either stick. :(

First of all, I'd like to make clear that I highly respect Airbus cockpit design and implementation of envelope protection.
Now to the question. The Airbus side stick is a lot stronger than a game joystick and very reliable, but is functionality-wise the same as a gaming joystick, just a stick loaded by a passive spring force sending back position signals. It had to be that way in the 90s when the sticks were designed, although we were already running digital force loops in real time @ 5000 Hz on a Motorola 68020, and had the technology to couple two sticks in software. Did it for Mercedes and for quite a few simulators, but sadly not for Airbus.
So how it could for instance be implemented for the A320 side stick: program the passive forces that are there now for normal operation but always couple the sticks, program a stick pusher for when in alternate law and stalling, and leave the current override button solution in for when the Control Loading System fails. We were part of an aircraft company then and making the system airworthy would not have been a problem.

Well, the spring provides all the feedback there is to provide. The side-stick does not command control surface position, it commands specific wing loading for pitch and roll rate for roll. So both position and force correspond to that command and there is no need for anything active. And springs are much simpler, and thus reliable, than active component would.

I agree, the design phylosofy of Airbus has done away with active feedback of flight parameters and for good reasons. But how is it not beneficial to directly feel what the other pilot is inputting on the stick?

@JanHudec The 777 yokes are FBW but provides force feedback. The difference being (afaik) that Boeing doesn't have the different control laws. I'm not sure how one would decide how to correlate the different control laws to stick resistance.

Yes the B777 has an onboard version of what in simulators is called the Control Loading System. Program any force characeristic you like, and couple multiple sticks in real time.

Only one pilot should be making input at any time. Pilot monitoring can see the input on their PFD and dual input is handled by warning and needs to be dealt with by better CRM anyway. Also because the side-stick is less accurate, the input is often series of brief nudges rather than smooth movement (especially in roll). The other pilot couldn't have their hand on the side-stick if it reflected that.

Yes during normal operation everything is very well thought out and operates smoothly; but in abnormal, high stress situations force coupling could provide additional intuitive feedback. All aircraft with mechanically coupled controls have the one pilot flying at a time stipulation, right?


text:
What is the ATC phraseology to request a parallel track?
What is the correct phraseology to request a parallel track due to weather for example?
Something like:

OE-DLM, request parallel track to the east, 2 nm

Background: I am asking for non-airline operations under IFR. I am using the GTN 750  and there is a feature to easily create a parallel track with a specified distance to the original route and I would like to use this feature. For example, when I'm in cruise flight, I see a weather cell and want to fly next to it by offsetting my route.

It is rather simple:

OE-DLM, request 2NM right of track.

However, weather circumnavigation is mostly done with headings. This way it is easier for the controller to predict your movement and thus preferred by many:

OE-DLM, request heading 120 for the next 20NM to avoid (weather).

This way it is easy for the controller to predict your movement and he has an idea where your avoidance will be completed. Additionally you are not forced to turn behind a cell you weren’t able to look behind before.
Mainly done in oceanic airspace where you try to make your way through the ITCZ (InnerTropical Convergence Zone) is this variant:

OE-DLM, request up to 20NM right of track to avoid (weather).

If approved you got a 20NM wide corridor to play in.
Flying offset is rather uncommon in the crowded European airspace (or generally every crowded airspace) but a normal thing everywhere else. China even uses it as a tactical separation method employed by the controller.

This is more often handled as heading changes than specific distances, you would do something like this:

Pilot: "XYZ Center, N12345 with request"
  ATC: "N12345, XYZ Center go ahead"
  Pilot: "XYZ Center, N12345 Requesting heading change 20 degrees right for weather"
  ATC: "N12345 heading change approved"

And then after you've cleared your weather, you would request your revised heading in the same manner. If you need to descend/climb you can make those requests in a similar way, ATC may "block off" some altitude for you.
Usually though commercial flights will have routed you around already probably before taking off, so I assume you are talking about something other than airline operations.

Airline operations don't have any magic tio avoid weather. The ever changing nature of weather means dispatchers can do their best to route airliners around weather, but pilot requests for routing around weather is very common.

Thank you, I have edited my question to add background.

@TomMcW No, they don't, and I agree that it is common, but at least with airliners on known routes, one or two at "the front" request the routing and ATC reroutes the others in the air. The ones on the ground are rerouted before they take off by the company to avoid holds and get the slot times right on arrival.


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Is it bad practice to say my callsign first in a transmission to ATC?
Usually ATC is saying the callsign of the plane he speaks with first and then starts with the actual transmission. Pilots usually say their transmission first and end it with their callsign.
However, I somehow started to say my callsign first before every transmission. For example:
Intended:

Me: Cleared for take-off runway 33, OE-CNK

How I do it:

Me: OE-CNK, cleared for take-off runway 33

Is this bad practice? Will it annoy the air traffic controller because he has to keep in mind my callsign for the whole transmission? Or is this not very relevant?

Yes, it's wrong.  You should start with the station you are calling first, whether ground or air.  "Lazytown tower, Bigfly31, request screaming landing followed by rapid taxi".

And whatever you do, don't mix up the protocol with this guy!

@TomMcW The Monterey Controller is doing a great job following the rules. On the other hand, some of the pilots  could use some remedial radio training.

@JScarry He's giving them their training:) Although he's definitely in the right, I'd bet in school that guy had a perpetual "kick me" sign on his back

@TomMcW But: "Audiatur et altera pars" - Seneca. The title of the video could also be "[FUNNY ATC] CARELESS PILOTS at Monterey MRY".

@Simon no it is not wrong, according to the edited question. On initial contact one should always first say the station being called, then the station doing the calling. But on subsequent contact the ATC unit is not stated (neither by the pilot nor by ATC). That doesn't change where the station ID should be in the message (the beginning). OP's edited question clarifies the context: readback of an instruction, i.e. post-initial-contact.

It depends on the country where you are flying, and also what the message is.
For example in the UK according to CAP 413 Chapter 2, section 2.48:

2.48
Ground to Air: Aircraft callsign – message or reply.
Air to Ground:

Initiation of new information/request etc. – Aircraft callsign then message;
Reply – Repeat of pertinent information/readback/acknowledgement then aircraft callsign.

In Germany the normal procedure is to start a message with the call sign, a reply may have the call sign at the end.  From the AIP, GEN section 3.4-9:

6.1 Procedures for Radio Communications
...
(4) The call sign shall be transmitted at the beginning of a message.
  A direct answer to a message may be terminated by the call sign
  instead.

In the United States, the FAA's Aeronautical Information Manual, section 4–2 "Radio Communications Phraseology and Techniques," states the following (emphasis in original):
4–2–3 a Initial Contact.

The terms initial contact or initial callup means the first radio call you make to a given facility or the first call to a different controller or FSS specialist within a facility. Use the following format:

Name of the facility being called;
Your full aircraft identification as filed in the flight plan or as discussed in paragraph 4-2-4, Aircraft Call Signs;
When operating on an airport surface, state your position.
The type of message to follow or your request if it is short; and
The word “Over” if required.

So on initial callup you should state the facility being called followed immediately by your callsign.
Then later in the section (emphasis mine):
4–2–3 c Subsequent Contacts and Responses to Callup from a Ground Facility.

Use the same format as used for the initial contact [...] The ground station name and the word “Over” may be omitted [...] Acknowledge with your aircraft identification, either at the beginning or at the end of your transmission, and one of the words “Wilco,” “Roger,” “Affirmative,” “Negative,” or other appropriate remarks [...]

Thus the proper procedure is:

Use your callsign at the beginning of any subsequent transmission (e.g. a request or report), but
When acknowledging an instruction (such as a takeoff clearance) you may use your callsign before or after the acknowledgement.


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Is Autopilot engaged when an engine failure above V1 occurs on modern airliners?
If you lose one engine during takeoff in a multi-engine aircraft, you're trained to follow a standard emergency procedure: Pitch for Vy, use bank and rudder for zero side slip, mixture full rich, props full forward, power full forward, gear and flaps up, and then run an emergency checklist for one engine failure.
When one engine failure happens in a modern jetliner, what do the pilots do? Do they engage the autopilot right away so that the computer can take care of pitch, bank, and rudder for the best performance for a single engine operation? Or do they basically follow the same procedure as the one written above, which is used by GA multi-engine pilots?

As a comment since the info is dated. In the 1990s in 747-100/200 aircraft the pilots would continue to hand fly the aircraft. If they decided to continue the flight rather than return to a landing, they might choose to engage the autopilot during cruise. Personally, I wouldn't do that, and I once hand flew from Santiago, Chile to Panama City, Panama when we lost an engine shortly after takeoff.

@Terry Did it use substantially more fuel on 3 engines?

@TomMcW Yes, indeed. We had fueled for a flight to Miami. Rather than dump fuel and return, it made more sense cost wise to burn the fuel to continue and get as far as we could, and that was Panama City. Our takeoff was at max gross, and we couldn't get high enough on 3 engines to clear the Andes on our original route. Thus not only were we down low with the higher associated burn rates, but our necessarily revised routing was far less direct. In Panama, we offloaded the cargo (it was a freighter), and then did a 3-engine ferry to Miami.

Most non-normal ops in an airliner follow the same concepts for light aircraft - make sure the aeroplane is under control and following the desired flight path before you do anything - which includes engaging an autopilot.
So for the Boeing aircraft I fly, ensure you've got a positive rate of climb, speed should be V2 - V2+15, ensure the aeroplane is in trim (pitch first, then rudder) and your following any prescribed engine failure routing.
In terms of autopilot use, part of our company SOP is to engage the autopilot at 1000ft AAL, this is not to relieve the pilot of the responsibility of maintaining control - it won't control the rudder - more to reduce the immediate workload and allow more effective management of the non-normal situation. Once the autopilot is engaged the aeroplane can be accelerated, flaps retracted and any non-normal checklist completed.

Does the yaw damper not have enough authority to handle an engine out?


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How often is the cabin air replaced in A320?
One of the A320 pilots told me that there is an icao rule that air should be replaced every 90 seconds?
Is this true ? can someone give me a supporting article for the same

Note that the fresh air inflow is mixed with recirculated air (probably the majority of the flow), and that mixed flow should achieve the entire cabin volume replacement over that time (or whatever the requirement is). I don't  think it means it's all replaced with fresh air every 90 sec.

Ok John still is there any figure for the same ? like I saw a video in which boeing says its done every 2-3 minutes

There will be something in FAR25 that specifies minimum air replacement rates.  https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11  I would go searching there.

A typical cabin is a volume of 150 m3, and air is not replaced but diluted with new air from the outside (bleed air for the A320 - more), while some air is rejected overboard at some rate by the outflow valves to stick to the cabin pressure ("altitude") schedule. Knowing the outflow valve rate is the key for any dilution computation. However air is filtered when recirculated if this is your point. ICAO doesn't deal with cabins, but FAA, EASA, etc do (+IATA as union).

Let's do some simplified arithmetic:

The packs airflow in an A320 is 1.2 kg/s per pack in HIGH setting, but let's take the normal setting 1 kg/s, thus 2 kg/s considering both packs
The cabin of an A320 is around 190 m^3
Let's assume the cabin altitude in cruise 8000 ft, i.e. the air density at 20°C is 0.897 kg/m^3
The total airmass of the cabin is then 190×0.897 = 170.43 kg
Assuming that the cabin pressure is constant in cruise (so all the air coming from packs is going out via the outflow valve), in 90 seconds we change the following mass of air: 2 kg/s * 90 s = 180 kg

So 170.43 ~= 180, then 90 seconds is a correct estimation.


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Did Concordes have de-icing boots?
Concordes were a supersonic passenger aircraft of which only 20 were built from 1965 to 1979 and were retired in 2003 for expense reasons and non-fuel efficiency. The plane was placed into regular service in 1976.
Did this aircraft have de-icing boots?

This question is similar to: Ice Protection System for supersonic aircraft. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem.

Yes, the Concorde had heated strips (not boots) on the leading edge, they are labelled 92 and 93 in the following cutaway (source):


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What is the map projection for FAA terminal procedure charts?
What is the map projection for FAA terminal procedure charts and what are the parameters (standard parallels) for each specific airport? I couldn’t find any information with the publication information and also not on the charts.

Most terminal procedure charts are not even to scale: "Generally, DP charts are depicted “not to scale” due to the great distances involved on some procedures or route segments." (Aeronautical Chart User’s Guide - Terminal Procedure Publications). In those cases, the map projection does not matter since the charts are more of a sketch anyway. I don't know what they use when the charts are to scale though.

@Bianfable You should post that as an answer.

@TomMcW Well, it doesn't really answer the question. Some charts are to scale and that means they must have a map projection. The document I found does not say which one they use...

Thank you @Bianfable, while this applies to most STARs and SIDs I guess, the IAPs should be to scale (and a projection?), as they are georeferenced when used in e.g. ForeFlight.

@TobiBS What do you mean by “they are geofeferenced?”

It means some applications like ForeFlight can display the chart overlayed on the moving map. The G1000 can do that do too, I think.

If it’s capable of doing that And the points line up properly then there are two possibilities: either the charts use the same projection as the moving map uses or at that scale there isn’t a significant difference. I would expect the latter.

Are you talking about EPSG:4326 projection for WGS84 coordinate planes?

According to the Aeronautical Chart Users’ Guide, page 95:

FAA Terminal charts are prepared in accordance with specifications of the Interagency Air Committee (IAC) and their supporting technical groups for the purpose of standardization, which are approved by representatives of the Federal Aviation
Administration (FAA), and the Department of Defense (DoD).

Looking up the IAC specifications we can find that for instance for Instrument Approach Procedure charts

Projection shall be Lambert Conformal, Polyconic, or Polar Stereographic.

Thank you so much!


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Mnemonics to distinguish popular airplane models
When in the airport I'd love to be able to distinguish most popular aircraft from each other, how can I say that a plane I am looking at is say a320 or 737 etc? They look so similar. I can detect 747 for 4 huge engines, but that's as far as I can go.

See https://www.airlinerspotter.com/

https://aviation.stackexchange.com/a/2742/1084
The easiest thing is looking at the fin for small types! The A320 from Airbus doesn’t have a dorsal fin, the B737 from Boeing has dorsal fin and the vertical- and horizontal stabilizer reaches to the far end of the fuselage.
The big ones are generally easy because the B747, A380, A340 and MD11 are distinguished by the number of engines and the hump (B747) for the half, full (A380) or none upper deck (A340).
The A350 is similar to the B787, but it helps that A350 wears a “black mask” around the cockpit. Therefore the A350 looks like Räuber Hotzenplotz or Zoro or whatever you prefer in your part of the world.
The landing gear is barley visible and different and swapable engine types are also difficult. The nose of Airbus planes is more round, while a nose from Boeing is usually more pointy.
For me the problematic ones are the A330 which is shorter than A350 and B777 which is longer. But there are also the B757 (small body) and B767 (wide body). For this ones you need probably take a close look and it helps if a different type is next to them, to get an impression of relative sizes.
A350 - Callsign Bandit?

Number of doors is also a good one. The A330 wing tips are shared only between A330, A340 and 747-400 (and since the latter two are explained above, anything with an upward kink in the wingtip that has two engines must be an A330). Also, for widebodies, to my eyes it’s the Airbusses which have the pointy noses and the Boeings with the blunt noses.

Wingtips are a good hint for the A330! Thanks.


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Which engines do airliners start first?
I noticed many planes, especially Boeings (correct me if I'm wrong anywhere), start engine "2" first and shut down engine "2" last.
However, I was watching an Airbus A350 cockpit video when I realised the A350 starts engine 1 first and shuts down engine 1 first. Hence, I have 2 questions:

In which order should the engines of the A350 be started and shut down?
What determines this order and are they the same/ similar on aircraft of the same manufacturer or can they differ within the same manufacturer as well?

Related: Why is engine No.2 started first, instead of No.1?

Depends on the operating procedure of the airplane... For example the primary brake system's hydraulic pressure could be connected to the nr 1 or nr 2 engine so depending on the aircraft one of the engines should be started first but which one it is depends on the aircraft type and systems being connected to that side.

@Jan At least on a Boeing, all hydraulic systems are pressurized (electrically or pneumatically) before starting any engine. Maybe it is different for the A350 though...

At least in the A320 the hydraulics can be unpressurized before engine start. Even though the A320 has two electric hydraulic pumps, one for the yellow and one for the blue system - the yellow pump is only running when the cargo doors are operated or when set on manually from the flight deck and the blue pump only kicks in when the first engine is started (or in flight as a backup). At least as far as I know... I'm only a simulator pilot not a real world airline pilot.

A good answer may include a diagram of hydraulics (or a link to it) for all the mentioned aircrafts (B737, A350, A320) highlighting the differences in pressurising means for all circuits.

For some general reasons, see Why is engine No.2 started first, instead of No.1?
Airbus A350
Specifically to the Airbus A350, I could not find any reason why one engine must be started first. As far as I know every modern airliner could start engines in arbitrary order.
The Airbus A350 has two hydraulic systems. From the Airbus A350-900 Flight Deck and Systems Briefing:

The A350 hydraulic system has two independent
  hydraulic circuits:

The YELLOW hydraulic circuit
The GREEN hydraulic circuit

Each system can be pressurized with an engine driven pump from either engine, or electrically on the ground:

Engine Driven Pumps (EDPs)
Four EDPs pressurize the hydraulic system. On each
  engine there are two EDPs, one pressurizes the
  GREEN hydraulic circuit and the other pressurizes the
  YELLOW hydraulic circuit. Thus if an engine fails, the
  remaining engine can still pressurize both hydraulic
  circuits.
Electric Motor Pump (EMP)
One EMP per hydraulic system can provide hydraulic
  pressure on ground only, when all engines are shut
  down. For example, the EMP of the YELLOW circuit
  operates automatically for cargo door actuation.

The main wheel brakes are powered by both hydraulic systems:

And also the parking brake uses pressure from both accumulators:

Which engine is started first has therefore no impact on which hydraulic system is powered first or which type of braking is available.
Airbus A320
The situation is a bit different for the Airbus A320. As Jan pointed out in the comments, it is normal to start the right engine first, because it provides hydraulic pressure to the yellow system. From the A320 FCOM Hydraulic chapter:

Green System Pump
A pump driven by engine 1 pressurizes the green system.
Blue System Pumps
An electric pump pressurizes the blue system. A pump driven by a ram air turbine (RAT) pressurizes this system in an emergency.
Yellow System Pumps
A pump driven by engine 2 pressurizes the yellow system. An electric pump can also pressurize the yellow system, which allows yellow hydraulics to be used on the ground when the engines are stopped. Crew member can also use a hand pump to pressurize the yellow system in order to operate the cargo doors when no electrical power is available.

The yellow system powers the alternate/parking brake system:

However, starting the left engine first is still perfectly possible. Hydraulic pressure from the accumulator is still available after pressurizing the yellow system with the electric pump before pushback to provide braking.
Boeing 737
The Boeing 737 has two hydraulic systems: A and B. Both can be powered by electric pumps. From the FCOMv2 13.20.2 Hydraulics - System Description:

Both A and B hydraulic systems have an engine–driven pump and an AC electric
  motor–driven pump. The system A engine–driven pump is powered by the No. 1
  engine and the system B engine–driven pump is powered by the No. 2 engine. An
  engine–driven hydraulic pump supplies approximately 4 times the fluid volume of
  the related electric motor–driven hydraulic pump.

The braking is provided by both systems. From the FCOMv2 14.20.3 Landing Gear - System Description:

Normal Brake System
The normal brake system is powered by hydraulic system B.
Alternate Brake System
The alternate brake system is powered by hydraulic system A. If hydraulic system
  B is low or fails, hydraulic system A automatically supplies pressure to the
  alternate brake system.
Brake Accumulator
The brake accumulator is pressurized by hydraulic system B. If both normal and
  alternate brake system pressure is lost, trapped hydraulic pressure in the brake
  accumulator can still provide several braking applications or parking brake
  application.

Again, there is no reason to start one engine before the other because of hydraulics/braking.
Conclusion
Engine start order is mostly tradition and will be specified by the airline op-specs. The shutdown oder also does not matter since both engines are usually shut down at (almost) the same time at the gate, except for a single engine taxi where the hydraulic system that powers the nose-wheel steering must be pressurized.

Interesting thing is, that if the A320 family is taxiing with just one engine, it uses #1 because only then it has brakes and nose wheel steering (except the PTU is used, which is usually avoided, afaik). See this answer

Boeing 787 is usually started Right then Left but the start process for both engines is concurrent. So the Commander calls START RIGHT, then opens the right fuel control switch, then immediately calls START LEFT and opens the left fuel control switch.


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How many flight control shutoff valves are there in Boeing 737?
I came to know from a source that there are total 6 flight control SOV 3 of which associated with each of system A and B hydraulic system .but in 737 FCOM under stand by hydraulic section in schematic diagram they have shown only 2 in total, 1 associated to each hydraulic system .so I am little bit confused .

The FCOM only shows one per hydraulic system, but my guess is that there is actually three (on each for Rudder, Elevator and Elevator Feel Computer, Aileron).


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Where can I get AMM for Boeing 737NG?
I am not connected to airlines .I am just an aviation enthusiasts .while studying the 737 FCOM I realized to study AMM also .but in Google I cant find it anywhere .so can anyone tell me how to get access to AMM ?

I’m voting to close this question because resource location is off-topic within the scope defined in the help center.

You should precise in the question what is AMM.

Buy it from Boeing.
Documents like the FCOM and the AMM are copyrighted and proprietary documents and aren't meant to be freely available.  Sometimes you'll find one on the internet that somebody posted, but they did so in violation of a license.
If you do find an AMM (or excerpts from it -- as far as I know the entire document is enormous) on the internet, changes are good that it's well out-of-date.
That said, there are better resources than the AMM to learn about the operational side of the 737.  As a pilot who flies it, the AMM isn't a document that I study or even have in the electronic flight bag.  It's for mechanics, and in some cases its limitations are significantly different than the limitations in the flight crew manuals (the company equivalent of the FCOM).  There are things that the mechanics can do on an as-needed basis that they don't want the pilots doing during routine ops, and so our limitation, for example, was in the 737-300, only one pack run off of APU bleed air for cooling.  The mechanics could run both packs for cooling, but when they did that it wasn't in order to cool the aircraft, it was to load up the APU and see how it responded to a high bleed air demand.  The first time I saw them do that was a "wait, you can't do that!" kind of moment, but their manual allows it.
Boeing, however, does NOT want pilots performing hybrid procedures, or "I saw a mechanic do this..." fixes.  Things that may work fine on the ground at the gate, may be a really bad idea to try in flight.  So Boeing has pilot procedures & limits for the pilots, maintenance procedures & limits for the mechanics, and just like in Ghostbusters, "don't cross the streams... it would be bad."

How does one go about buying it from Boeing? A simple email? Surely they require more than just currency to obtain this manual?


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Is it safe to use a propeller plane to fly commercially over the Tibetan Plateau?
I saw this youtube video today: Flying over Mt Everest. A lovely view of stunning nature - quite spectacular. However, I looked up the plane - or at least what I think is the plane - and found that it might be a propeller plane: 9N-AHV (I saw this registration number in the video).
The plane appears to be a BAe Jetstream 41 - check that link for details - it is Wikipedia.
Of particular interest (maybe):

Service ceiling: 26,000 ft (7,900 m)
Service ceiling on one engine: 15,000 ft (4,572 m)

Tibetan Plateau: "...average elevation exceeding 4,500 metres (14,800 ft)".
So it sounds like an engine failure for this plane type means that it  can not descend to the acceptable service ceiling? The service ceiling is the maximum usable altitude of an aircraft.
This seems unsafe?
I am not a pilot, but I hear it is rare to fly over Himalaya / Mount Everest / Tibet because planes can't go low enough for passengers to breathe without oxygen if there is an emergency? If they lose an engine they have to descend I read. There is also a lot of turbulence. Why Planes Don't Fly Over Himalayas.
Airports: Lhasa and Kathmandu - short runways - not many places for emergency landings due to medical emergencies and such. And they wrote "the place is desolate with little people around". That sounds a bit funny seeing as the area is situated between China and India :-). Remote it is though. And China has 94% of its population in the eastern part of the country (towards the sea).
Links:

https://www.quora.com/Why-don%E2%80%99t-planes-fly-over-Tibet/answer/Paul-Oroborust

Maximum altitude limit where propellers can propel?

How high can propeller airplanes fly?

From my experience driving through Tibet I can tell you that 4500m really isn't enough. The highway routinely climbs over 5000m to get to the other side of a mountain. What people call "Tibetan Plateau" most of the time is central Tibet around Lhasa where it's high but almost flat and 4500m might be OK. But on the east side, no, at 4500m you will hit something pretty quickly.

And no, there are not a lot of fights over Tibet because there are not a lot of people traveling there (and not a lot of people living there, either). As a result there's only one major airport near Lhasa. (At least 9 out of 10 people I know considers it "hazardous" or "risky" even flying to Lhasa, where it's "only" 3600m). Tibet is a beautiful place as long as you can handle the elevation.

"Is it safe...?" is highly subjective.  Every flight carries risk.   In fact, every day you're alive carries risk.  Most flights over the Plateau would be perfectly safe.  I think the valid way to phrase the question is, "Does the flight meet aviation rules for that area?"   Aviation rules will govern things like minimum altitude, glide range, fuel reserves, single-engine performance, etc.  Either the flight meets those rules, or it does not.

It is obviously less safe than flying over flat sea level terrain in good weather, with multiple divert airfields available.  The points you raise in your question are valid and indicate that you have a solid grasp of risk assessment.  So that makes me wonder what kind of an answer you seek?

Pardon the beginner question: are service ceilings given above sea level? here there is a relevant mass of land at an average altitude >2000m, I guess it should be taken into account, or am I wrong?

@EarlGrey It is given in Pressure Altitude, which is as a first approximation equal to altitude above mean sea level (AMSL). Local weather phenomena can influence local pressure though.

@Bianfable thanks, then there is even a certain risk of overestimating the ceiling: at the mount Everest height (~8850m) the presure is 253 mm Hg, but the standard atmosphere equation predicts 263 mm Hg.


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Areodynamics of ancient flying machines. Did they really worked for them?
If any modern day aerodynamic scholars by chance  gone through rig vedas of hindu mythology. It has been described like below.

"jumping into space speedily with a craft using fire and water ...
  containing twelve stamghas (pillars), one wheel, three machines, 300
  pivots, and 60 instruments."

Reference from: https://en.wikipedia.org/wiki/Vimana
Is there any scientifical proofs that those aerodynamic principles would have worked for them or just a myth?

Mythology doesn't need scientific foundations, but it's easy to claim there are some, as narrative are not very detailed, so many interpretations are possible. But that's not scientific either. It's like Nostradamus predictions.

Just a few points:

You are yourself conceding that these are from mythology. If something is from mythology, it is (almost) certainly a myth.
Description of a fictional flying machine and explaining their principles of operation are completely different. To the best of my knowledge, none of these texts even try to explain the principle of operation.
Actually, there was a study carried out on one of these texts, which thoroughly debunked the claims made and concluded,

... the planes described above are (at) the best poor concoctions, rather than expressions of something real.None of the planes has properties or capabilities of being flown; the geometries are unimaginably horrendous from the point of view of flying; and the principles of propulsion make then  resist rather than assist flying.